Levodopa is not absorbed from the stomach, but the stomach plays an important role in controlling how levodopa reaches its absorptive sites in the small bowel. Some medicines, including dopamine agonists and anticholinergics, can also delay gastric emptying, as can severe stomach acidity, although over-treatment of this problem can also prevent dissolution of levodopa tablets, leading to incomplete absorption.

Moreover, gastric emptying can be delayed by Parkinson’s itself or by constipation caused by the colon-gastric reflex. Levodopa tablets may remain in the stomach for a long time, leading to delayed absorption in the small intestine and a delayed response to the treatment.

An enzyme called dopa-decarboxylase that is present in the stomach lining can convert levodopa trapped in the stomach into dopamine, making it unavailable to the central nervous system. Furthermore, dopamine formed in the stomach may stimulate gastric dopamine receptors, leading to stomach relaxation and reduced gastric motility, and this can worsen the problem.

Liquid levodopa may improve motor fluctuations by ensuring better absorption. Levodopa methyl ester and dispersible formulations are absorbed more quickly than standard preparations, especially when taken after meals. Subcutaneous infusion of the dopamine agonist apomorphine is effective in controlling motor fluctuations by bypassing the gastrointestinal tract.

People with Parkinson’s should try to eat small meals and take medicines when fasting. Dopaminergic D2 receptor antagonists, such as domperidone, improve gastric emptying in Parkinson’s. Similar effects can be induced by the 5-HT4 receptor agonists mosapride, presumably by increasing local release of the neurotransmitter acetylcholine.

http://www.epda.eu.com/en/parkinsons...owel-problems/